If you thought a one-meter rise in sea level by the end of the century, as predicted in many studies, is really bad, climate scientist Dr. James Hansen has some disturbing news: If emissions continue on their current course, we could actually be looking at one-meter sea level rise by mid-century, and multi-meter rise by 2100.

Speaking on a teleconference to the media abouthis latest paper written with eighteen co-authors, Dr. Hansen explained that most climate models, including those reflected in IPCC reports, understate the problem of sea level rise. Melting ice contributes to sea level rise, but the meltwater from the ice can also have additional feedback effects that accelerate further melting. Since IPCC models do not include the positive feedback effects of meltwater on ocean systems, these models underestimate sea level rise.

Models that include this positive feedback suggest that the ice sheet response to warming will be “nonlinear.” Some data suggest that the rate of ice sheet melting could double every 10 years or 20 years (the available data is too limited now to be more exact). If ice sheet melting has a 10-year doubling time, we would see one meter sea level rise in 50 years (20-year doubling would take about 100 years), with multi-meter sea level rise after another 10 or 20 years.

When Hansen’s team included watermelt in their models, they learned that, even though only a “tiny, tiny fraction of the ice sheets” have melted, nonetheless the melting is already having important, measurable effects. Hansen explained that in the North Atlantic, signs suggest that meltwater from Greenland is beginning to shut down the circulation of Atlantic ocean waters. This very early stage of shutdown may explain warming ocean temperatures measured along the northeast coast of America, sea levels rising along North America faster than global averages, and cooling temperatures measured southeast of Greenland. In the Southern Hemisphere, meltwater from Antarctica may be resting on the ocean surface, preventing warmer waters from rising and giving off their heat, thus causing subsurface ocean warming, and accelerating ice shelf melting.

The insights from this new paper derive not just from changes to models, but also stem from improved systems for measuring ice sheets and ocean temperatures. In recent presentations to Citizens’ Climate Lobby, Hansen described new data collection methods that provide more accurate measurements. GRACE satellites measure ice sheet mass, providing a better idea about where and how fast ice is melting. In addition, roughly 3,000ARGO floats measure ocean temperature and salinity at different depths. With meltwater’s positive feedback factored into their model, Hansen’s team can explain the real world’s accelerated melting (currently also measured by GRACE satellites), and can account for temperature changes in surface and subsurface ocean temperatures (measured by ARGO floats).

Where do we go from here? Although Hansen offered compelling explanations for why Greenland and Antarctic ice is melting faster than IPCC reports forecasted, even his model underestimates the rate at which the ice is melting. He acknowledged that models need further refinement and more measurements need to be collected. He believes that if he and his colleagues improve their modeling of “small scale” ocean mixing, then the model will generate more accurate forecasts of ice melting rates. But this will take time.

Time is the issue. Hansen concluded his press presentation by emphasizing that “this is a complex story, but one with important practical implications.” He stressed that the public needs to worry less about how minute details of the paper will be analyzed by experts, and worry more about whether we are approaching a point of irreversible harm. The public, media and Congress need to ask scientists whether we have reached a point of no return with dangerous implications for our children.

Time is the bottom line. Even though scientists do not know with certainty every detail of what is happening, if we wait until models are perfect before deciding to cut emissions quickly, feedback loops may lead to irreversible ice melt, multi-meter sea level rise, disruptive population resettlement, expensive infrastructure abandonment, coastal city relocation, and more dangerous superstorms.

A long time ago, Hillel provided a different type of model — one for ethical decision-making: “If I am not for myself, who will be? If I am only for myself, what am I? If not now, when?” We learn from Hansen’s research, there may only be our current time’s when. We grasp that if we are only for ourselves, our children won’t be able to be for themselves . . . and we wrestle with what that would make us.